The prion concept, according to which proteins alone can be infectious, represents a new paradigm in biology. Such protein-only infectious agents are believed to be responsible for transmissible spongiform encephalopathies, a group of fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle. Mammalian prions propagate by a mechanism involving the conformational conversion of a largely a-helical cellular prion protein, PrPC, to misfolded, [unreadable]-rich amyloid-like aggregates, PrPSc. More recently, the prion hypothesis has been extended further to include the phenomenon of protein conformation-based inheritance in yeast and other fungi. Particularly puzzling features of such conformational infectivity and inheritance are the existence of so-called prion strains linked to discrete disease phenotypes within the same host species, and their role in transmissibility barriers which frequently prevent efficient prion transmission between different species. While recent studies indicate that prion strains originate from the ability of proteins to misfold into multiple distinct conformers in forming self-propagating amyloid aggregates, molecular-level understanding of these phenomena has been hampered by the paucity of high-resolution structural data. In this project we aim to advance the understanding of fundamental aspects of protein conformation-based inheritance and infectivity by providing detailed insights into the molecular mechanisms and structural basis of amyloid propagation and transmissibility barriers for a non-infectious Y145Stop PrP variant (PrP23-144). This C-truncated PrP mutant is associated with hereditary cerebral amyloid angiopathy in humans and exhibits in vitro some of the most fundamental aspects of PrP propagation including the phenomena of prion strains and species barriers. The specific aims focus on the determination and validation of a detailed structural model for human PrP23-144 amyloid aggregates, and elucidation of the structural differences related to the emergence of distinct PrP23-144 amyloid strains with unique transmission characteristics (i.e., seeding specificities) linked to mutations of two critical amino acid residues near the C-terminus of PrP23-144. Modern multidimensional solid-state NMR spectroscopy will be the primary experimental structural technique employed in the study. In addition, we will use hydrogen/deuterium exchange solution NMR methods, atomic force microscopy and tilted-beam transmission electron microscopy. PUBLIC HEALTH RELEVANCE: Prions are infectious proteins thought to cause transmissible spongiform encephalopathies (TSEs), a group of fatal neurological conditions that include Creutzfeldt-Jakob disease in humans, "mad cow" disease in cattle, chronic wasting disease in deer and elk, and scrapie in sheep. The phenomena of prion strains and transmission barriers, both related to the structural characteristics of prion aggregates, are two critical features underlying the propagation mechanism of prions and, indeed, other amyloid-like protein aggregates. Understanding these phenomena with atomic level detail will provide novel insights about the molecular basis of prion and amyloid disorders and is of fundamental importance for public health.